1. Field of the Invention
The present invention relates to direction detecting devices, and more particularly to a direction detecting device used for a car navigation system.
2. Description of the Prior Art
A direction detecting device uses either a stand-alone type sensor or a GPS (Global Positioning System) receiver which receives an electric wave from a GPS satellite, and measures the position of a movable body in which the GPS receiver is installed. In order to more precisely perform the position measurement, some direction detecting device simultaneously uses both a stand-alone type sensor and a GPS receiver.
Such a direction detecting device using both the GPS receiver and the stand-alone type sensor includes a GPS receiver, a geomagnetic sensor (attitude sensor) for sensing geomagnetism, and a microcomputer. The GPS receiver receives an electric wave from a GPS satellite and generates first direction data from the received electric wave. The geomagnetic sensor detects a horizontal component of very weak geomagnetism approximately equal to 0.3 Gauss, and generates second direction data corresponding to a direction in which a movable body moves. The microcomputer receives the first direction data and the second direction data, and produces direction data therefrom.
When the above-mentioned direction detecting device is installed in an automobile, the following problems occur. A body of the automobile has been slightly magnetized due to geomagnetism, and hence generates a very small magnetic field. As a result, the geomagnetic sensor detects a composite vector of the geomagnetism and the very small magnetic field derived from the vehicle body. Consequently, the second direction data includes an error component. Since the magnetic field derived from the vehicle body is semi-fixed, a fixed component of the magnetic field is detected by making one revolution of the vehicle body, and a correction (rotation correction) is carried out using the above fixed component. In this manner, the error component in the second direction data can be eliminated. However, when the automobile travels in a place where a strong magnetic field is generated, such as a railway crossing, another magnetization takes place and thereby the magnetic field is changed. Hence, the second direction data cannot be accurately calibrated in the above-mentioned manner. In order to perform accurate navigation, the above-mentioned rotation correction must be carried out again. However, this forces the user to have a large load due to the repeated performance of the rotation correction. Further, the output signal of the geomagnetic sensor varies if the detected horizontal components of geomagnetism have differences between position to position.